The present invention relates to a modular electrode system, and its use, for facilitating the introduction of a macromolecule into cells of a selected tissue in a body or plant. The modular electrode system comprises a plurality of needle electrodes; a hypodermic needle; an electrical connector that provides a conductive link from a programmable constant-current pulse controller to the plurality of needle electrodes; an impedance meter; and a power source. In a preferred embodiment of the present invention, an operator can grasp the plurality of needle electrodes that are mounted on a support structure and firmly insert the them into the selected tissue in a body or plant. The macromolecules are then delivered via the hypodermic needle into the selected tissue. The programmable constant-current pulse controller is activated and constant-current electrical pulse is applied to the plurality of needle electrodes. The applied constant-current electrical pulse facilitates the introduction of the macromolecule into the cell between the plurality of electrodes. Cell death due to overheating of cells is prevented by limiting the current to levels that do not cause excessive heating.
Broadly, electroporation is the use of a transmembrane electric field pulse to induce microscopic pathways (pores) in a bio-membrane. These pores are commonly called “electropores.” Their presence allows macromolecules, ions, and water to pass from one side of the membrane to the other. Thus, electroporation has been used to introduce drugs, DNA or other molecules into multi-cellular tissues, and may prove to be an effective for the treatment of certain diseases. However, the use of electroporation in living organisms has several problems, including cell death that results from generated heat and the inability of electropores to reseal. The beneficial effects of the drug or macromolecule are extremely limited with prior art electroporation methods where excessive cell heating and cell death occurs.
To better understand the process of electroporation, it is important to look at some simple equations. When a potential difference (voltage) is applied across the electrodes implanted in a tissue, it generates an electric field (“E”), which is the applied voltage (“V”) divided by the distance (“d”) between the electrodes.E=V/d 
The electric field intensity E has been a very important value in prior art when formulating electroporation protocols for the delivery of a drug or macromolecule into the cell of the subject. The field intensity is inversely proportional to the distance between the electrode in that given a voltage, the field strength increases as the distance between the electrodes is decreased. However, a caveat is that an electric field can be generated in a tissue with insulated electrodes (i.e. flow of ions is not necessary to create an electric field). Although not wanting to be bound by theory, it is the flow of ions that opens the electropores and allows movement of molecules into the cells of a subject during electroporation. The flow of electric charge in a conductor or medium between two points having a difference in potential is called the current. The current between electrodes is achieved by the ions or charged particles in the tissues, which can vary among tissues and patients. Furthermore, the flow of conducting ions in the tissue can change between electrodes from the beginning of the electric pulse to the end of the electric pulse.
When tissues have a small proportion conducting ions, resistance is increased, heat is generated and cells are killed. Ohm's law expresses the relationship between current (“I”), voltage (“V”), and resistance (“R”):R=V/I 
The resistance in the tissue between two electrodes varies depending on the charged particles present therein, thus, the resistance in the tissue changes from the beginning of the electric pulse to the end of the electric pulse.
Heating is the product of the inter-electrode impedance (i.e. combination of resistance and reactance and is measured in ohms), and is proportional to the product of the current, voltage and pulse duration. Heating can also be expressed as the square of the current, and pulse duration (“t”, time). For example, during electroporation the heating or power (“W”, watts) generated in the supporting tissue can be represented by the following equation:W=I2Rt 
Broadly, prior art teaches that metallic electrodes are placed in contact with tissues and short pulses of predetermined voltages are imposed on the electrodes initiating the cells to transiently open membrane pores. The protocols currently described for electroporation are defined in terms of the resulting field intensities E, which are dependent on short pulses of voltage proportional to the distance between the electrodes, and regardless of current. Accordingly, the resistance or heating cannot be determined for the electroporated tissue, which leads to varied success with different pulsed voltage electroporation protocols. Certainly, the difference in upper limit amplitudes of a voltage pulse between electroporation protocols that facilitate effective electroporation and electroporation protocols that cause the cells to die are very small. Additionally, a definite correlation has been observed between death of cells and the heating of cells caused by the upper limit amplitudes of the short voltage pulses. Thus, the over heating of cells between across electrodes serves as a principal cause for the ineffectiveness of any given electroporation voltage pulsing protocol. Furthermore, the current between electrodes serves as a primary determinant of the effectiveness of any given pulsing protocol, not the voltage across the electrodes.
When electricity is delivered to the cells of a subject, the dose of electricity can be accurately described in terms of charge (“Q”), which is the current (“I”) and the time (“t”), according to the formula:Q=It
If the current is not constant, as is the case in prior art electroporators, Q represents the time integral of I. In this respect, charged particles, be they ions or molecules, behave in a similar fashion. For example, when silver ions are deposited on an electrode to define the standard unit of electrical charge (the coulomb), only the charge, as defined above, is of importance. A certain minimum voltage must be present to generate a current, but the quantity of ions deposited can not be determined from a predetermined voltage. Correspondingly, the quantity of charged particles delivered to cells in an electroporator can not be derived from the voltage imposed on the electrodes.
Although electroporation is widely used for laboratory gene transfection and gaining increased importance for non-viral gene therapy, it is generally employed using trial-and-error optimization schemes for lack of methods to predict electroporation's effects on cells (Canatella P J, Gene Ther 2001 October;8(19):1464–9). For example, it has been shown that the efficiency of plasmid gene transfer to skeletal muscle can be significantly improved by the application of an electrical field to the muscle following injection of plasmid DNA. However, this electrotransfer is associated with significant muscle damage that may result in substantial loss of transfected muscle fibers (McMahon J M, Signori E, Wells K E, Fazio V M, Wells D J Gene Ther 2001 August; 8(16):1264–70). The reduction of the voltage used in the technique can result in a decrease in muscle damage, with a concomitant reduction in expression, but without a significant decrease in the number of transfected fibers.
The effectiveness of electroporation is limited by the fact that there is a threshold value for the pulse intensity below which electroporation does not occur, and an upper limit above which the cells are destroyed.
Experimental evidence shows that the difference between the upper and lower limits is so small that it is very difficult to design effective pulsing protocols without undue experimentation. This makes use of the technique difficult.
References in the art directed toward an electroporation apparatus illustrate the usefulness of both an electrode apparatus and an in vivo method of electroporation. Correspondingly there are many U.S. Patents that claim either specific electrodes, or methods for electroporation. For example, U.S. Pat. No. 6,302,874 is a method and apparatus for electrically assisted topical delivery of agents for cosmetic applications; U.S. Pat. No. 5,676,646; is a flow through electroporation apparatus for implanting molecules into living blood cells of a patient; U.S. Pat. No. 6,241,701 & 6,233,482 describes a method and apparatus for electroporation mediated delivery of drugs and genes. More specifically they describe a method and apparatus for electroporation therapy (“EPT”) for treating tumors treated by a combination of electroporation using the apparatus of the invention and a chemotherapeutic agent caused regression of tumors in vivo; U.S. Pat. No. 6,216,034; describes a method of programming an array of needle electrodes for electroporation therapy of tissue; U.S. Pat. No. 6,208,893; describes an electroporation apparatus with a connective electrode template; U.S. Pat. No. 6,192,270; Describes an electrode assembly for an apparatus and a method of trans-surface molecular delivery; U.S. Pat. No. 6,181,964, describes a minimally invasive apparatus and method to electroporate drugs and genes into tissue. Using electroporation therapy (“EPT”) as described in the invention, tumors treated by a combination of electroporation using the apparatus of the invention and a chemotherapeutic agent caused regression of tumors in vivo; U.S. Pat. No. 6,150,148, describes an electroporation apparatus for control of temperature during the process, by generating and applying an electric field according to a user-specified pulsing and temperature profile scheme; U.S. Pat. No. 6,120,493, describes a method for the introduction of therapeutic agents utilizing an electric field electroporation apparatus; U.S. Pat. No. 6,096,020, describes an electroporation method and apparatus generating and applying an electric field according to a user-specified pulsing scheme; U.S. Pat. No. 6,068,650, describes a method of selectively applying needle array configurations for in vivo electroporation therapy; and U.S. Pat. No. 5,702,359, describes an electrode apparatus for the application of electroporation to a portion of the body of a patient with a sensing element for sensing a distance between the electrodes and generating a distance signal proportionate to the distance between said electrodes, and means responsive to said distance signal for applying pulses of high amplitude electric signal to the electrodes proportionate to the distance between said electrodes. All of these cited patents are hereby incorporated by reference
The aforementioned patent disclosures along with many others describe electroporators and methods for use by utilizing a predetermined voltage between the electrodes. Because the impedance between electrodes that are embedded in a tissue can vary from case-to-case, or tissue-to-tissue, a predetermined voltage does not produce a predetermined current. Thus, prior art does not provide a means to delineate the exact dosage of current to which the cells are exposed and limits the usefulness of the electroporation technique. For this very reason, conventional electroporators generate tremendous amounts of heat is tissues that can easily kill cells. For example, a typical electronic 50 ms pulse with an average current of 5 Amperes across a typical load impedance of 25 ohms can theoretically raise the temperature in tissue 7.5° C., which enough to kill cells. In contrast, the power dissipation decreases in a constant-current system and prevents heating of a tissue, which reduces tissue damage and contributes to the overall success of the procedure.
The difficulties present in prior-art electrodes stem from the fact that the pulse energy is concentrated in the center of the array, the point where the material to be transfected is deposited.
As a result, the spatial distribution of energy delivery assumes a very non-uniform character. Therefore, only a fraction of the cells in the volume encompassed by the electrode assembly is electroporated.
Thus, there is a need to overcome the problems of prior art by providing a means to effectively control the dosage of electricity delivered to the cells in the inter-electrode space by precisely controlling the ionic flux that impinges on the conduits in the cell membranes.